1. Field of the Invention
The present invention relates to a solid-state imaging apparatus and a driving method thereof.
2. Description of the Related Art
In recent years, a CMOS image sensor of a column A/D (column parallel A/D) type in which A/D (analog-to-digital) converters are arranged correspondingly to each of columns to pixels arranged in a matrix shape has been used as a solid-state imaging apparatus. There is an A/D converting technique which is used in the column A/D. For example, there is an A/D converting technique called an integrating type or a ramp comparing type in which an analog signal is compared with a reference signal changing in a slope, a counting process is executed in parallel with the comparing process, and a digital signal is obtained based on a count value at a point of time of completion of the comparing process. In the case where a differential comparator is used for the comparing process in the integrating type A/D conversion, by executing a resetting operation of the comparator before a comparing operation, an offset that is caused by a variation in threshold values of transistors can be eliminated. However, there is a problem that if the resetting operation is insufficient, an electric potential at a signal input terminal is out of a range of the reference signal, the comparator is not inverted during the gradual change of the reference signal or is inverted just after a start of the gradual change of the reference signal.
According to the technique disclosed in the Official Gazette of Japanese Patent Application Laid-Open No. 2006-340044, there has been proposed a method whereby, first, a reference signal is input to a differential comparator, a reference signal unit of the reference signal is reset, and thereafter, a current source array for an offset is provided in order to add an offset voltage to the reference signal changing in accordance with time.
However, according to the Official Gazette of Japanese Patent Application Laid-Open No. 2006-340044, there is such a problem that a circuit scale is large. This is because the current source array for the offset, for changing the reference signal of the differential comparator to a predetermined voltage after the resetting, is necessary apart from a current source array, for a slope which is used for the comparing with the analog signal at the time of the A/D conversion. Since a start voltage of the reference signal is started from a voltage changed by the current source array for the offset, there is a case where noises or a voltage fluctuation of the current source array for the offset appears as noises which are common in each column.